I haven't tested this, but this is how I would write it based on what you have provided.

#!/usr/bin/perl
use strict;
# split-logfiles
# you don't need to test if it is open
#$isopen = 0;
my $myfile = "/var/tmp/test.log";
# no reason to do this testing
# unless($isopen and -f $myfile) {
# open the happy file here, but you may want to use >> if
# you need to append rather then overwrite on each run
open MYHAPPYFILE, ">$myfile" or
die "Error opening $myfile";
# don't need to test if it is open because we
# are dying if it isn't
# $isopen = 1;
# }
while (my $log_file = <STDIN>) {
printf MYHAPPYFILE "%s", $log_line;
}
exit 0;